1. Field of the Invention
The present invention relates to an electrophotographic photoconductor, in particular, to an electrophotographic photoconductor exhibiting improved image quality and reduced likelihood of causing ghost phenomena due to exposure.
2. Description of the Related Art
Image formation using an electrophotographic system is diversely applied to copiers, printers, plotters and complex digital imaging machines combining the functions of these machines in an office, and recently also to small-sized printers and facsimile machines for personal use. Many types of photoconductors for these electrophotographic apparatuses have been developed since the invention by Carlson (U.S. Pat. No. 2,297,691). Photoconductors these days generally use organic material.
There is a type of photoconductor, known as a functionally separated photoconductor, which consists of an undercoat layer, a charge generation layer, a charge transport layer, and, as required, a protective layer, these layers being laminated on a conductive substrate. The conductive substrate can be made of aluminum or the like. The undercoat layer can be, for example, an anodized film or a resin film. The charge generation layer may contain an organic pigment exhibiting a photoconductive property, such as phthalocyanines or azo pigments. The charge transport layer contains a molecule having a partial structure that involves hopping conduction of charges, such as a molecule of amine or hydrazone that bonds with conjugated π electrons. Another type of known photoconductor, a single layer type photoconductor, comprises a photosensitive layer exhibiting both charge generating and charge transporting functions and a protective layer that are laminated on an undercoat layer.
Each layer composing the photoconductor is normally formed, because of mass-production, by dipping and coating a conductive substrate in a coating liquid prepared by dissolving or dispersing a pigment, a charge generation agent, to exhibit a charge generation or light scattering function, or a charge transport agent to exhibit a charge transport function.
In a so-called reverse development process that is primarily employed in recent electrophotographic apparatuses, an exposure light source uses a semiconductor laser or a light emitting diode with an oscillation wave length ranging from 450 nm to 830 nm; digital signals of a picture or characters are transformed into optical signals; the light is irradiated on an electrified photoconductor to form an electrostatic latent image on the photoconductor surface; and the latent image in turn is visualized by toner.
Phthalocyanines, among charge generation agents, have been extensively studied because the phthalocyanines have larger light absorbing capability in the oscillation wave length region of semiconductor lasers than other charge generation agents, thus exhibit excellent charge generation ability. Known photoconductors use a variety of phthalocyanines having a central atom of copper, aluminum, indium, vanadium, or titanium (see Japanese Unexamined Patent Publication S53-89433; U.S. Pat. No. 3,816,118; Japanese Unexamined Patent Publication S57-148745; and U.S. Pat. No. 3,825,422).
Electrical characteristics of a photoconductor having laminated organic films are controlled by the contact conditions between a charge generation substance and a charge transport substance included in different layers at the interface between the layers, as well as by the properties of each layer. The injection characteristic of carriers in particular, is affected by the structure of the interface.
If charge injection from a charge generation layer into a charge transport layer is inhibited due to inhomogeneity in the interface structure and the charges are accumulated around the interface, an image defect such as so-called image memory appears. Therefore, it is important from the viewpoint of image quality to achieve an adequate interface structure. If a photoconductor surface including an inadequate interface structure is once exposed, charge accumulation occurs at the interface between the charge generation layer and the charge transport layer in this region. When the photoconductor surface of this region is electrified afterwards, the charges accumulated in the vicinity of the interface are released, or the photo-induced carriers generated in the charge generation layer are deactivated, which causes a ghost phenomenon upon exposure. Negative memory occurs in the case of excessive carriers for neutralizing surface charges, and positive memory occurs in the case of deficient carriers.
In order to improve resolution of an image, a charge transport agent with low mobility is often selected for suppressing lateral movement of holes in the photoconductor, or the concentration of the charge transport agent in the films is controlled to be low.
However, if a charge transport agent with low mobility is selected, the temperature dependence of the photoconductor surface potential increases, and if the concentration of the charge transport agent is reduced, a drawback of increased residual potential arises in addition to the above mentioned defects. These defects further increase the ghost phenomena caused by exposure.